I've built Gentoo on a SheevaPlug (arm) and I am emerging various packages.

I'm new to working on a non standard platform and do not know if I should post problems with an emerged package in the forum Gentoo on Alternative Architectures, or in the forum Portage and Programming.

And should I also (or only) log a bug when a package fails to emerge, or does that depend on whether it is masked or not?

This is a help-forum, not a bug-reporting-forum
If you think you have found a bug in an ebuild, your best bet is to report it on Gentoo Bugzilla after reading the Gentoo Bug Reporting Guide.
If you think you have found a bug in the code of the program you are trying to compile then you should probably report it to upstream.

How ever...if you need help in maybe understanding what went wrong with your emerge then these forums can be the right place, but we will require a little bit more information. The snippet you posted is not the error, but the exit message from emerge. This message is displayed twice, once just below the actual error and once as part of delayeed messages from emerge at the end of the emerge run.

What the guys here will need to be able to figure things out are the real error messages from the compile of the code.